Very low frequency posts on radio frequency, signal processing, and engineering academics

Friday, February 26, 2016

Particular things about indoor particulate matter

I’ve been doing some reading and hearing from experts on indoor air quality (or lack thereof) as part of a new NIH project on IoT tools for pediatric asthma research. Particularly in Salt Lake City this time of year, there’s a lot to be concerned about regarding our outdoor air quality. I hadn’t realized before I started this project how little I know about indoor air quality. I thought I’d summarize some things I’m learning for people like me who are not experts in this area.

Here’s the predominant way to measure air quality. Particulate matter (particles suspended in air) with diameter 2.5 μm or less is abbreviated PM2.5. There are lots of things smaller than 2.5 μm, some of them benign, some of them known to be harmful, and some of them researchers just don’t know whether they’re harmful or not. It is too difficult to separately measure every single type of pollutant, so we instead typically measure the collective total PM2.5, which there are standard and relatively inexpensive ways to measure. It is typically measured as total mass per volume, given in units of μg/m3. Some inexpensive devices measure simply count the number of particles per unit volume, which they say is proportional to the mass per volume.

PM2.5 certainly affects our health. There have been many studies which show the association of PM2.5 exposure with particular pulmonary and cardiovascular diseases. Taking these results in combination, researchers believe that reducing PM2.5 by 10 μg/m3 increases human lifespan by 6 months, on average [Pope2009]. The US EPA specifies a limit for PM2.5 -- your average exposure to PM2.5 shouldn’t exceed 12 μg/m3, and shouldn’t exceed 35 μg/m3 during any day. However, PM2.5 levels measured in the average home is above this EPA limit [Polidori2006, Logue2011], and certain activities (cooking, cleaning, burning candles) can raise the PM2.5 level tens to hundreds of times higher those thresholds.

In contrast with outdoor air quality, there’s a lot you can do to improve your home air quality. You don’t need an act of congress or to convince everyone to bike to work. Many people could do a few things (and not do a few things) that would improve their home indoor air quality, starting today.

Here are some things you can do to improve your indoor air quality:

Use a high efficiency air filter in your HVAC system. “High efficiency” means that they’re good at removing particulate pollution, not that they save energy. Filters are rated on the “MERV” scale, and higher is better for removing particulates (the MERV acronym is funny, but what can you expect, it was made up by engineers in the group ASHRAE). Good filters are in the 11-16 range of the MERV scale. Really high efficiency filters can make it hard for your furnace to circulate air through your home, so they tend to make them wider, like 2” or 4” wide, instead of the 1” wide regular filters. These cost more, but don’t need to be replaced as often, so it ends up being the same average expense per time, and then you don’t need to remember as often. Of course, you need to be sure that your furnace filter space can handle a filter wider than 1”, or get a HVAC repair person to “make” space for your wider filter.

Use the vent when you cook, if you have a vent that vents air to the outside. Cooking is huge source of PM2.5, particularly frying or using the oven at a high temperature. Many people only have a range hood that puts the air back in the house -- if that’s you, sorry, it won’t help. Also note that stove hoods are much more efficient at removing the air from your pan on the back burner, compared to the front burner.

Remembering to do stuff is hard. So here are some things that will improve your air quality by NOT doing them.

Don’t smoke (duh!)

Don’t light candles or incense.

Don’t burn wood in your fireplace.

In short, don’t burn stuff in your home. Also,

Don’t use an ozone-producing “air purifier”. Ozone is bad for you, why use a product that contributes ozone to your indoor air? Ozone-producing air purifiers include those that use the words “ionize”, “UV”, or “electrostatic”. If you want a standalone air purifier, get one that pulls air through a filter.

Restrict your use surface cleaners or air fresheners. In particular, they produce compounds that react with ozone to produce formaldehyde, which is a carcinogen [Nazaroff 2008]. After using them, rinse the cleaner off the surface with water, as the problem is the reaction between the surface cleaner on your surfaces and the ozone in the air.

Don’t humidify with tap water. The water droplets you spread into the air contain the mineral ions which came in your tap water. When the water evaporates, the minerals stay suspended in the air and contribute to the PM2.5. Also too high of humidity will increase mold and dust mite levels.

If your household includes very young or very old, or someone with heart or lung conditions, you should consider using a standalone HEPA air purifier, which can cut in half the PM2.5 near it [Batterman2012] or turning on the fan from your forced air system, when you’re home.

As I said, I’m no expert in this area, so please just use this as a starting point. If you have comments or corrections, let me know.